An image sensor is a semiconductor device for converting optical images into electric signals, and is mainly classified as a charge coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor.
The CMOS image sensor includes a photodiode and a MOS transistor in each unit pixel, and sequentially detects the electric signals of each unit pixel in a switching mode to realize images.
A CMOS image sensor according to the related art includes a photodiode area for receiving an optical signal and converting the optical signal into an electrical signal, and a transistor area for processing the electrical signal.
However, in the CMOS image sensor according to the related art, photodiodes and transistors are horizontally disposed, and the photodiodes are positioned on the same plane as that of a transistor circuitry.
Although the CMOS image sensor according to the related art overcomes the disadvantages of the CCD image sensor, problems still remain in the CMOS image sensor.
In other words, in the CMOS image sensor according to the related art, the photodiodes and the transistors are positioned on a substrate such that they are horizontally adjacent to each other. Accordingly, additional areas are required for the photodiodes. For this reason, a fill factor area may be reduced, and resolution may be restricted.
Further, the CMOS image sensor according to the related art has difficulties in optimizing the manufacturing process when the photodiodes and the transistors are simultaneously manufactured. In other words, although a rapid transistor manufacturing process requires a shallow junction in order to achieve low sheet resistance, the shallow junction is unstable for the photodiode manufacturing process.
In addition, in the CMOS image sensor according to the related art, since additional on-chip functions are provided in the CMOS image sensor, the size of a unit pixel must increase in order to maintain the sensitivity of the image sensor, or the area for the photodiode must decrease in order to maintain the size of a pixel. However, if the size of a pixel increases, the resolution of the CMOS image sensor may decrease, and if the area for the photodiode decreases, the sensitivity of the image sensor may decrease.